When evaporating sulfate black liquor to high dry solids content (60-75%), the viscosity of the liquor increases rapidly. At the same time its evaporability decreases significantly because the black liquor adheres to the heating surface thus impairing the heat transfer to the black liquor. In order to achieve a high caloric value for combustion of the black liquor in a recovery boiler, the dry solids content should be as high as possible.
Co-pending U.S. patent application Ser. No. 312,424, the entire disclosure of which is herein incorporated by reference, suggests a method for decreasing the viscosity and improving the evaporability of sulfate black liquor. This is brought about by raising the temperature of the black liquor above the cooking temperature in a reactor so as to split the macro-molecular lignin fractions contained in the liquor.